Climatically induced spatial sedimentation dynamics of late Holocene sediment infill (Laguna Potrok Aike, southern Patagonia, Argentina) – an aeral sediment survey in the framework of the ICDP project PASADO

KASTNER, S.; OHLENDORF, C.; HABERZETTL, T.; LÜCKE, A.; MAIDANA, N. I.; MAYR, C.; SCHÄBITZ, F.; ZOLITSCHKA, B.

Postdam, Alemania

Congreso; ICDP International Continental Scientific Drilling Program - Deutschland Kolloquium; 2009

ICDP

The 100 m deep and max. 770 ka old maar lake Laguna Potrok Aike (51°58’S, 70°23’W) has a high potential as a palaeolimnological key site for the reconstruction of terrestrial palaeoclimate conditions within a region of scarce palaeoenvironmental archives (Zolitschka et al., 2006). The strong influence of variing southern hemispheric wind and pressure systems on the lacustrine deposits was proven by various interdisciplinary multi-proxy studies of the lake´s sediments as well as a climate modelling approach (Wagner et al. 2007). Therefor, the lake holds a unique record of palaeoclimatic and palaeoecological variability within the dry steppe environment of south-eastern Patagonia (Fig. 1). Hydrological variations of the lake are closley related to the fluctuations of the Southern Hemispheric Westerlies (Mayr et al., 2007) as it is shown from depositional changes inferred from the lacustrine sediment sequence as well as subaerial and subaquatic lake level terraces (Anselmetti et al., 2008). For this reason the lake was chosen as an ICDP drilling site in 2008 within the “Potrok Aike maar lake sediment archive drilling project” (PASADO). Geochemical, palynological, diatomological and isotopic investigations were carried out with high temporal resolution on a 18.9 m long sediment record covering the last 16,000 years (Haberzettl et al., 2007; Mayr et al., subm.; Wille et al., 2007). Beyond this one point source of information the present study focuses on the understanding of internal depositional dynamics which control the characteristics and spatial distribution of the sediment infill of the lake. Furthermore, it provides information improving the accuracy of the interpretation of the long sediment record recovered within the PASADO project. A grid of 46 gravity cores of up to 49 cm length was established in 2005 to survey the spatial sediment distribution. This grid of cores covers a range of water depths from 9 to 100 m (Fig. 2). All 46 cores were scanned with X-ray fluorescence technique and for magnetic susceptibility with 1 and 4 mm spatial resolution. Using Ca and Ti as well as magnetic susceptibility data the existing cores were correlated and linked to the established depth/age model (Haberzettl et al., 2005). As these parameters vary considerably and not consistently within the suite of littoral cores, a correlation prior to the 2005 sediment surface is solely based on cores from water depths exceeding 45 m. Thus, samples of the surface sediments were taken from all 46 cores while sub-sampling of selected time intervals - AD 1960, 1800, 1610, 1500, 1380 - was only possible for up to 26 well correlated cores from the deep central basin. These time slices cover palaeoenvironmental distinctive intervals representing different hydrological settings (i.e., different lake levels, Haberzettl et al., 2005). The sediment was further analysed geochemically (for element concentrations of C, N, S, and total phosphorus), sedimentologically (grain size), palynologically, diatomologically, and for stable isotopes of organic matter (C, N) and carbonates (C, O). Subsequently, distribution maps for all parameters and for each time slice were compiled by kriging methods. Additional 16 gravity cores and 40 shoreline sediment samples from all around the lake’s perimeter were taken during the PASADO fieldwork in autumn 2008 to complement the existing grid. First results of the surficial sediments, i.e. representing the last 20 years of the record, confirm pronounced differences between the littoral cores down to 45 m water depth and the lake’s profundal cores separated from each other by steep slopes (Fig. 2; Kastner et al., in prep.). Modern sedimentation patterns of grain size, benthic diatoms, total inorganic carbon, Ti and Ca point to distinct internal depositional dynamics induced by the dominant westerly winds. At the eastern shore frequent erosion, resuspension and redistribution of littoral sediment is followed by transport to a profundal accumulation area (Fig. 3). Hence, sedimentation within this terminal lake is not only influenced by lake level changes, episodic inflows and the surrounding geology but also by wind driven wave action and resulting internal currents. The sub-recent spatial sediment distribution is evaluated and interpreted in the context of the modern processes. Due to core correlation the interpretation is restricted to information of the deep central basin area. Changing wind patterns (Mayr et al. 2007; Meyer and Wagner, 2008) and varying lake levels (Haberzettl et al., 2005) are assumed to cause modifications of depositional dynamics and affect the varying palaeo-shoreline proximity to the analysed sediment cores during the selected Holocene time sections (i.e., Little Ice Age, Medieval Climate Anomaly and transitional periods). To evaluate the mechanisms of the sediment distribution during different hydrological settings the grainsize data seem to be an important parameter as this data explicitly indicate sediment reworking from the lake margins as well as influences of the discontinuous tributaries (c.f. Fig. 3). Additional parameters complement and differentiate the underlying mechanisms. Distribution patterns in the deep basin reveal intensified sediment redistribution during lake level low stands and strengthend winds during post-Little Ice Age time (1960). In contrast, Little Ice Age (AD 1800) conditions of a lake level high stand and less intense westerly winds result in a more homogeneous sediment distribution within the deep central basin. Furthermore the spatial sediment distribution indicate distinct influences of the main western, the north-eastern and the south-eastern tributaries and the north-eastern outflow.